The background of the invention might best be understood through the discussion of a problem which exists in F.sub.1 F.sub.1 frequency repeaters. An F.sub.1 F.sub.1 frequency repeater is a device wherein a weak radio signal of a given frequency is received and a signal of the same frequency but greater amplitude is retransmitted. The broadcast and received signals utilize the same antenna, the transmitter and receiver being connected to said antenna through the use of a four port hybrid. Thus the situation is one wherein the receiver and transmitter are connected to opposite ports of the four port hybrid and the antenna is connected to one of the other two ports. The remaining open port has in the past been terminated by a fixed impedance which compensates for the impedance of the antenna. The system operates in the following manner.
A signal from the transmitter is introduced at the transmitter port. There the signal divides into two signals 180 degrees out of phase, one of which is routed toward the antenna port and the other of which is routed toward the load port. The largest portion of the signal reaching the antenna port travels out the antenna into free space. A secondary portion of this signal, however, is reflected back into the antenna port. This reflected signal travels to the receiver port. During the same time period a portion of the signal which has been coupled to the load port is reflected to the receiver port. It is desirable to minimize the signal reflected from the antenna port which enters the receiver port in an F.sub.1 F.sub.1 repeater since a signal of sufficient amplitude may lead to a condition of oscillation. To prevent this condition the impedance at the load port is judiciously selected to simulate the impedance at the antenna port so that the signals reflected to the receiver port are equal in magnitude but 180 degrees out of phase and will therefore cancel. Operation of the system therefore is degraded when the antenna port is connected to an unstable impedance such as an antenna swaying in the wind. As a result of this instability the two signals arriving at the receiver port will not be of the same magnitude and 180 degrees out of phase and the introduction of an undesired signal into the receiver will result. If this signal is of sufficient magnitude the oscillation condition mentioned previously may occur.
One object of this invention is to automatically maintain the proper amplitude and phasing of the signal reflected from the load port in order to provide for maximum cancellation at the receiver port while the antenna port impedance varies randomly. A second object of the invention is to provide a repeater system which simultaneously broadcasts and receives at essentially the same frequency while utilizing a single whip antenna.